循环变形下镁合金孪生变形与微结构演化耦合机理及本构关系研究

To reveal coupling relationships between twinning deformation and microstructure in Mg ally, the distribution of microstructure characteristics is represented by adopting statistical analysis methods, and these variables of microstructure characteristics are integrated into the mechanism of twinning-detwinning deformation through the functional relationships among grain size, crystal orientation, grain boundary misorientation and resolved shear stress, twinning strength. And then the new model of meso-plastic cyclic constitutive is established. On this basis, the numerical simulations of macro and meso plastic behavior for Mg alloy under various cyclic loading are carried out by establishing a 3D voxel-based model of polycrystalline aggregation which reflects a real microstructure to reveal the coupling mechanism between twinning-detwinning deformation and micro-structure evolution． The main researches are that 1) An experimental study is carried out on observing the distribution of microstructure, such as grain size, crystal orientation, grain boundary misorientation of magnesium alloys. And the quantitative statistical relation between twins and microstructure is analyzed. 2) Through the energy principle and the dislocation movement mechanisms, the parameters of grain size, crystal orientation and grain boundary misorientation are integrated into the intrinsic mechanism of energy dissipation during twin nucleation with strain energy so as to present an energy criterion for triggering twin nucleation. 3) Both the coupling mechanism between twinning-detwinning deformation and microstructural evolution of magnesium alloys under cyclic deformation and the inherent relationship between the microstructure evolution and the macro-plastic behavior are investigated numerically.

为了揭示镁合金孪生变形机制与材料微结构之间的耦合关系，采用统计分析方法表征微结构特性分布，并建立晶粒尺寸、晶向、晶界倾角与孪生面分解剪切应力、孪生强度的函数关系，将这些微结构特征耦合到孪生-去孪生变形机制中，进而建立新的镁合金循环塑性本构模型。在此基础上，基于体素方法建立反映材料真实三维微结构的多晶集合体模型对各种循环加载条件下镁合金材料的细观和宏观塑性行为进行数值模拟研究，从而揭示孪生-去孪生变形与微结构演化的耦合机理。主要研究：1）试验观测镁合金晶体中晶粒尺寸、晶向、晶界倾角等微结构的分布，并分析各种类型的孪晶与微结构的数量统计关系。2）通过能量原理和位错运动机制，将晶粒尺寸、晶界倾角、晶向等参数与应变能统一到孪晶形核的能量耗散机制中，从而建立孪生形核发生的能量准则。3）数值研究镁合金循环加载下孪生-孪生变形与微结构之间的耦合机理，以及微结构演化与宏观塑性行为的内在联系。

为了揭示镁合金孪生变形机制与材料微结构之间的耦合关系，本项目开展完成了针对镁合金（AZ31、AZ61等）材料在单调加载、循环加载情形下的塑性变形行为的试验研究，然后对变形前后的镁合金试样开展微结构观测与统计试验。进一步基于细观本构理论，发展了融合晶粒尺寸、晶向、晶界倾角等微结构参数的孪生变形强度演化模型，并基于体素方法构建的三维微结构多晶模型，针对镁合金在加载变形试验条件下的塑性变形行为展开大量的数值模拟。通过对大量微结构观测数据及大规模模拟数据进行统计分析，得到如下结论：.1、镁合金多晶体中晶粒大小、孪晶厚度、晶界倾角等微结构分布基本符合高斯概率分布特征。.2、在单调加载变形中，大晶粒、高Schmid因子、小角度晶界有利于孪生形核，而孪晶长大对微结构的敏感度较弱。孪晶长大更多地受到晶内存在的孪生形核数和孪晶变体类型数量的强烈影响。.3、在不同应变量条件下，孪生形核数随晶粒大小的增加出现递增现象，但晶粒单位面积的形核数随晶粒面积的增加而呈现递减关系，同时晶粒大小对孪晶长大的影响较小。从总的孪晶体分数来看，总孪晶体分数随晶粒直径的增加而呈现递减趋势。.4、与取向相关的Schmid因子与孪晶变体体分数存在指数函数关系，呈现出一种强相关性，但不是所有高Schmid因子的孪生变体都能够形核长大。.5、随着晶界倾角的增加，孪晶体分数都呈递减趋势，但晶界角越大，越有可能承担更大等效应力，产生更多的应变。.6、在各种循环加载情形下，孪晶厚度与晶粒大小未存在明显的晶粒尺寸效应，但孪晶形核数量与晶粒面积呈稳定的正相关联系，且随应变幅值的增加，同样面积的晶粒产生的孪晶形核数也越多。.本项目揭示的孪生变形与微结构耦合关系，发展了镁合金本构模型，为高强高韧镁合金新材料的性能设计、生产工艺提供科学的理论依据。